Idiopathic pulmonary arterial hypertension (IPAH) is a lethal disorder characterized by pulmonary vasoconstriction and remodeling, leading to progressively worsening right ventricular hypertrophy, and eventually right heart failure. The familial form of IPAH is usually due to mutations in the type 2 receptor for the bone morphogenic protein pathway, BMPR2. BMPR2 can signal through several different pathways, including SMAD 1/5/8 and both p38 and p42/44 MAPK. The BMPR2 mutations in human IPAH patients frequently appear to leave SMAD signaling intact, whereas mutations leading to constitutive activation of both p38 and p42/44 MAPK are common. This strongly suggests that, in vivo in human patients, it is loss of BMPR2-mediated suppression of MAPK, rather than loss of SMAD signaling, that leads to the pulmonary hypertensive phenotype. We therefore hypothesize that it is dysregulation of p38 and p42/44 MAPK signaling through BMPR2 which leads to defects both in vasoreactivity and in pulmonary vascular structure, the central hallmarks of PAH. The purpose of this proposal is to directly test this hypothesis in transgenic models of PAH. To do this, we have developed inducible smooth muscle-specific mouse models of PAH which have defects primarily in vasoreactivity (BMPR2-delx4+) or in pulmonary vascular structure (BMPR2-R899X). We intend to (aim 1) determine whether either or both of the phenotypes can be prevented or treated with pharmacologic p38 or p42/44 MAPK inhibitors, (aim 2) determine the molecular pathways that link BMPR2 to elevated p38 and p42/44 MAPK, and (aim 3) determine the molecular consequences of aberrant MAPK signaling through BMPR2, using smooth muscle cells cultured from these animals. Relevance: Evidence from human pulmonary arterial hypertension patients suggests that defective MAPK signaling through BMPR2 causes disease. This study will determine how and whether defective MAPK signaling results in these problems, and will attempt interventions to effect prevention or treatment. We will do this using new mouse models which replicate both the mutations and the central characteristics of human disease, as well as cells cultured from these animals, with the goal of developing more effective therapies.